1. Field of the Invention
The present invention relates to the field of computer memory management units, and more specifically, to mapping less than 32 bits onto a 32-bit memory address bus.
2. Prior Art
In most computers, a central processing unit (CPU) communicates directly with both an address bus and a data bus. These buses are coupled to a memory system in addition to numerous other items, such as input/output ports, specialized processors, DMA units, etc. The new generation of microcomputers of today utilized single chip CPUs such as the 8086, 80386, 68000 and 68020. More recent chips, such as the 80386 and the 68020 utilize 32 bit address signals to access various locations within the memory.
Memory management units are well-known in the prior art and are used to provide efficient utilization of the computer's main memory. These units perform housekeeping functions, such as remapping, and often include a memory which stores data containing relocation of an address base and providing paging functions. Because of the complexity of present day CPU chips, more complex memory management units are provided to perform extensive and complicated memory management functions. One such chip is the 68851 paged memory management unit by Motorola Inc. to support the 68020 chip.
However, until the advent of the 32-bit microprocessor chips the earlier CPUs operated on 16-bit and 24-bit addressing schemes. Considerable software, including operating systems, have been written to run on these prior art 16-bit and 24-bit computers. Prior art memory management units operating in conjuction with these earlier CPUs are not able to provide the extended addressing bit capability of the new 32-bit processors. Although 32-bit memory management units are available, such as the aforementioned 68851, such units are very complex, costly and provide significantly more complex functions than the basic memory remapping which is required to convert the prior art 16-bit and 24-bit address ranges into a 32-bit address map.
For example, the Macintosh.TM. computer sold by Apple Computer Inc. of Cupertino, Calif., provided a 24-bit address scheme, wherein 24 bits physically addressed the memory. A newer computer operating on the 68020 CPU is now capable of addressing considerable more memory space due to its 32-bit address configuration. However, to run the earlier software written for the 24-bit machine, the 32-bit system must be capable of converting the 24-bit address range of the older system to a 32-bit address range of the newer computer system. Although other memory management units are capable of performing this function, the memory remapping of 24 to 32 bits can be accomplished much more simply and economically with the present invention.
The present invention builds upon those prior art memory management units, as well as the more recent 32-bit memory management units. The memory mapping unit of the present invention is simplistic in design and function and is economical from a cost stand point. The memory mapping unit of the present invention is capable of providing a 32-bit address range to physically access the memory by using the more recent 32-bit addressing scheme, or alternatively, converting the prior art address scheme having less than 32 bits to a 32-bit address.